A hot-rolled steel strip (hereinafter also simply referred to as a steel strip) is manufactured by rolling a heated slab such that the slab has a desired size. In this case, the hot-rolled steel strip is cooled using cooling water (water cooling) by a cooling apparatus during hot rolling (rough rolling, finish rolling) or after finish rolling. The purpose of this water cooling is to mainly control deposit or transformation structure of the steel strip and to regulate the quality of material so that intended strength, ductility, and the like can be obtained. In particular, accurately controlling at a predetermined temperature in the cooling after finish rolling is important in manufacturing hot-rolled steel strips having intended material properties without variation.
In recent years, as a result of soaring rare metal costs, methods have been developed that improve mechanical properties by transformation structure control based on cooling instead of alloy composition regulation. In the above-described water cooling, there is a great need for wide range control of cooling rate in response to requirements for material quality. In a typical run out table in the manufacturing of a hot-rolled steel strip, arrangement of pipe laminar nozzles for the upper surface and spray nozzles for the lower surface is often used as a cooling apparatus. The amount of cooling water is about 0.4 to 1.0 m3/min·m2 per one surface. For a steel strip having a thickness of 3 mm, a cooling rate of about 50 to 70° C./s is provided.
Recently, as regards hot-rolled high tensile strength steel, there has been a great need for further increasing cooling rate and aggressively performing transformation structure control. On the other hand, steel strips used, for example, for automotive bodies are sometimes soft steel strips and are formed into complicated shapes from the viewpoint of design or the like. Such steel strips are often required to have workability such as ductility rather than strength. If the cooling rate is too high, this workability may be impaired. So, such a cooling technique that cooling rate can be largely changed using the same cooling apparatus is required.
As regards hot-rolled steel strips, the passing performance of steel strips varies depending on, in particular, thickness. Unfortunately, difficulties occur. As regards high tensile steel for automobiles, most of steel strips have thicknesses from about 1.2 to 3.0 mm. In particular, a thin steel strip having a thickness of about 1.2 mm has poor stiffness and provides high passing speed. Accordingly, if the steel strip is passed while a large amount of cooling water is poured, the steel strip tends to bound or loop due to fluid resistance. So, a technique for reducing the amount of cooling water only when the thickness is small is also needed.
As described above, there is a great need for a technique for controlling cooling rate/amount of cooling water in order to control the size and target material of a steel strip. In response to this, there is, for example, a cooling technique described in Patent Literature 1.